Digital filter design

[engcad]

Hi everyone 

I'm new to labview and want to design a strain indicator using labview 

my signal conditioner give me a linear dc output with +/- 2.5mv noise amplitude 

I searched alot to know the right filter to use, finally a commercialy strain indicator use the following filter" 

Filter: Integrated linear phase FIR Sinc5 filter followed
by a Sinc3 filter with a programmable decimation rate.
Software selectable output rate provides >120 dB rejection
of 50 or 60Hz and higher level harmonics." 

Is this filter is suitable to my case? and what tools, VIs i can use to do it?

thanks alot 

[johnsold]

The first thing to do in selecting or designing a filter to to define what needs to be filtered. You mention the noise amplitude but did not indicate the signal amplitude range. If the signal is 100 V the problem is much different from the case where the signal is 10 microvolts!

You also need to specify the frequency content of the desired signal and the noise or interfering signal.  How fast can your strain change? Is the "noise" broadband white noise or primarily power line frequency interference?  The quote you included appears to refer to a process which attempts to remove power line frequency interference.

What will you be doing with the strain signal? Display only, automatic control? Is the signal (and noise) acquired continuously or in finite batches? How many samples do you read at a time? What is your samping rate?

Lynn

[engcad]

Thanks for the useful notes

the amplitude signal range 0 - 10 volt

I can't give a certain value for the strain change rate, but it is a static measurement so it's a small rate (1-2 mv/s)

I don't know what causes the noise signal

the signal is acquired continuously 

I use this measurement for automatic control

I can't determine the proper sampling rate 

Unfortunately I'm a mechanical engineer and new in this field 

May be I can't express my problem. Simply I want a stable reading like a commercial strain meter.

thanks 

[johnsold]

Somewhere in your data acquisistion setup is the sampling rate. Without knowing how you are doing the acquisition, it is hard to suggest where to look for the information.  Posting your VI (with some data saved as default) would be helpful.

Most likely the commercial meter does some kind of averaging or smoothing to create a "stable" reading.  It is easy enough to so those things in LV, but you need to know about the data acquisition rates and the automatic control update rates to determine appropriate parameters.  For example if the control system updates 10 times per second but you "stabilize" your readings by averaging all the data for a minute, the control system will be updating on stale data because of the averaging time.  You may need to use different processing for display from what is used for control.

Lynn